(1) Field of the Invention
This invention relates to radiographic apparatus for use in the medical field, and in industrial fields for performing non-destructive examinations, RI (Radio Isotope) examinations and optical examinations.
(2) Description of the Related Art
A conventional apparatus of this type includes a C-shaped arm supporting an X-ray tube at one end thereof and an image intensifier at the other end. The C arm is rotatable about the body axis of a patient or object under examination, i.e. the axis of a scan center shaft. The X-ray tube and image intensifier are thus rotatable together about the patient or object to scan it and obtain sectional images thereof (as disclosed in Japanese Unexamined Patent Publication 2001-45374 (page 3 and FIG. 1), for example).
However, the apparatus disclosed in the above patent publication has the following drawback. This drawback will be described with reference to FIG. 1. In FIG. 1, reference 101 denotes a C arm, 102 an X-ray tube, 103 an image intensifier, and M a patient. The C arm 101 is slid to rotate about the body axis (y-axis in FIG. 1) of patient M only through a range corresponding to the length of C arm 101. Since the movement of X-ray tube 102 and image intensifier 103 is limited to the range corresponding to the length of C arm 101, the C arm 101 cannot make one complete, continuous rotation about the axis of the scan center shaft (body axis). Thus, the X-ray tube 102 and image intensifier 103 are rotated about a sectional axis (z-axis in FIG. 1) in order to acquire sectional images of the entire patient M. This sectional axis is a different axis not parallel to the axis of the scan center shaft but extending through a site of interest of patient M. The X-ray tube 102 and image intensifier 103 are supported such that an X-ray beam center linking the X-ray tube 102 and image intensifier 103 is inclined at a predetermined angle α relative to the sectional axis (the angle α being called hereinafter “tomosynthetic angle”). The scan center axis and the sectional axis usually are set substantially perpendicular to each other.
The C arm 101 supporting the X-ray tube 102 and image intensifier 103 is required to have a sufficient length corresponding to an amount of sliding movement, and becomes heavy accordingly. In practice, therefore, the X-ray tube 102 and image intensifier 103 are not rotated directly about the sectional axis. The C arm 101 supporting the X-ray tube 102 and image intensifier 103 is rotated about the axis of an arm shaft (x-axis in FIG. 1), and the X-ray tube 102 and image intensifier 103 are rotated about the body axis of patient M (scan center axis). Thus, the X-ray tube 102 and image intensifier 103 are rotated indirectly about the sectional axis. This arm shaft is substantially perpendicular to the scan center shaft, and substantially perpendicular to the sectional axis.
(I)
Since the two shafts (scan center shaft and arm shaft) are rotated, it will take time (e.g. about six seconds) to rotate the X-ray tube 102 and image intensifier 103 about the sectional axis. Conversely, where the C arm 101, X-ray tube 102 and image intensifier 103 are arranged, respectively, to be rotatable directly about the sectional axis, the entire C arm 101 must be rotated about the sectional axis. Where the C arm 101 is rotated about the body axis of patient M (scan center axis), there is little chance of the C arm 101 colliding with the patient M. Where the entire C arm 101 is rotated about the sectional axis, the C arm 101 could collide with the end in the direction along the body axis, such as the head or feet, of patient M, and thus a rotating scan cannot be carried out safely.
(II)
Since the two shafts (scan center shaft and arm shaft) are rotated as noted above, the rotation of X-ray tube 102 and image intensifier 103 about the sectional axis results in an inconvenience that resolution along the sectional axis is lower than resolution along the other axes (e.g. the scan center axis). This is because the direction along the sectional axis corresponds only to a main scan in this invention, and low spatial resolution along the sectional axis results in anisotropic spatial resolution. Moreover, a high-speed scan is impossible because of the construction of the C arm.
On the other hand, a conventional X-ray CT apparatus has been developed to be capable of a continuous helical scan at a rate of about 0.5 sec. per rotation. However, this is still inadequate for imaging of a fast-acting internal organ such as the heart. A still image of the heart is reconstructed by combining ECG synchronous data collected from numerous angles during numerous rotations made for the same slice. Furthermore, it is impossible to realize isotropic spatial resolution because of the limitation of a simple helical scan.